A massive Multiple Input Multiple Output (MIMO) system consists of multiple remote radio units (RRUs). These RRUs can be sourced by a centralized digital transceiver unit that can be located a few kilometers away. Since there is hardware on the RRU that needs to be controlled according to the data being sent, there needs to be a mechanism to transfer control information from the digital unit such that the hardware complexity at the RRU does not increase too much.
Hardware on the RRU includes (but is not limited to) a power amplifier (PA), switches, automatic gain controls (AGCs), antenna calibration units, and antenna systems.
Current RRUs are not massive MIMO and, hence require significantly less hardware cost for control. However, the current solutions may not be scalable for massive MIMO.